1. Field of the Invention
The present invention relates to laser thermal processing, and in particular relates to apparatus and methods for performing laser thermal processing with laser diode radiation.
2. Description of the Prior Art
Laser thermal processing (“LTP”) (also referred to as “laser thermal annealing”) is a technique used to anneal and/or activate dopants of source, drain or gate regions of integrated devices or circuits, to form silicide regions in integrated devices or circuits, to lower contact resistances of metal wiring coupled thereto, or to trigger a chemical reaction to either deposit or remove substances from a substrate.
Various devices for performing LTP of a semiconductor substrate have been known and used in the integrated circuit (IC) fabrication industry. LTP is preferably done in a single cycle that brings the temperature of the material being annealed up to the annealing temperature and back down in a single cycle. If a pulsed laser is used, this requires enough energy per pulse to bring the entire chip or circuit up to the annealing temperature. Because the required field size can exceed four (4) centimeters-squared (cm2) and the required dose can exceed one (1.0) Joules/cm2, a relatively large, expensive laser is required. It is also difficult to achieve good dose uniformity over a relatively large area in a single pulse because the narrow spectral range of most lasers produces a speckled pattern due to interference effects.
Laser diode bars are well-suited to serve as a source of radiation for performing LTP because their wavelengths of 780 nm or 810 nm are readily absorbed in the top layer (i.e., ˜21 microns) of silicon Diode bars are also efficient converters of electricity to radiation (˜45%).
U.S. Pat. No. 6,531,681 (the '681 patent) describes how a linear laser diode array, or several linear diode arrays, can be used to form a uniform, narrow line image that can be scanned across a substrate to thermally anneal integrated circuits thereon. The '681 patent also describes how the line image can be placed on a mask and imaged through a projection system to process selected areas of a substrate scanned in synchronism with the mask. However, performing laser thermal processing with a linear array of laser diode bars as described in the '681 patent is problematic. Applications involving silicon substrates have system requirements (i.e., image width and dwell time) that require relatively high energy densities (e.g. In the range of 1300 W/mm2 for a 200 μs dwell time).
U.S. Pat. No. 6,747,245 describes the use of a P-polarized CO2 laser beam incident at near Brewster's angle to perform LTP of a silicon substrate with integrated circuits formed thereon. As described therein, the use of incident angles at or near Brewster's angle produces very uniform heating of substrates that are spectrally non-uniform at normal incidence. For example, at normal incidence bare silicon has a reflectivity greater than 30% and silicon oxide has a reflectivity of less than 4%. One benefit of using a CO2 laser when performing LTP is its ability to deliver a well-collimated beam having relatively high energy density. Another benefit is that the 10.6 μm wavelength emitted by the CO2 laser is large compared to the various film thicknesses likely to be found on a wafer ready for the annealing step. Small variations in film thickness therefore do not result in large variations in reflectivity as would be the case for a shorter annealing wavelength.
However, the CO2 laser wavelength of 10.6 μm is best suited for annealing heavily doped silicon substrates, which can absorb sufficient radiation in the top 50 to 100 μm of material. However, for annealing lightly doped substrates or substrates that are doped only in a shallow layer near the top surface, the CO2 laser radiation passes right through with very little of the incident energy resulting in useful heating.
Laser diodes, on the other hand, emit radiation at wavelengths of 780 nm or 810 nm. These wavelengths are readily absorbed in the top 10 to 20 μm of a silicon wafer. Thus, with laser diodes operating at the short time scales (i.e., 100 μs to 20 ms) associated with LTP, the heating depth is determined by thermal diffusion rather than by a long absorption depth (length).
It would therefore be useful to have systems and methods for performing laser thermal annealing at or near the Brewster's angle with polarized laser diode radiation delivered at relatively high energy densities.